Triathlon fixed

the jack of all trades thing isn't exactly true. Most triathletes I know are good at one sport, but not quite good enough, and reasonable at the other two, so think 'hey i know I'll do a triathlon!'
Most seem to come from running/ swimming backgrounds.

I myself took to it from a swimming background after being a good swimmer and then suffering a bad injury and never being competetive afterwards, and still to this day the majority of my training is running, because I'm totally the wrong build for it + find it evil in the extreme. Riding my fixed bike sorted out any problems with the cycling leg.

I'm crap at all three, but it's fun

It's odd that you say that "most seem to come from running/swimming backgrounds". That doesn't make sense as the cycle is 50% of the race, the run 30% and swim 20%. Would be better to come from a cycling background. No?

I split my weekly exercise into those percentages:
5 x commutes by bike (actually 10 sessions, but who cares, it's cycling, it's good)
3 x runs
2 x swims
That seems to work for me